Enhanced geothermal system

The Australian government has provided research funding for the development of Hot Dry Rock technology. Projects include Hunter Valley (1999), Cooper Basin: Habanero (2002), Cooper Basin: Jolokia 1 (2002), and Olympic Dam (2005).[75]

The EU's EGS R&D project at Soultz-sous-Forêts, France, connects a 1.5 MW demonstration plant to the grid. The Soultz project explored the connection of multiple stimulated zones and the performance of triplet well configurations (1 injector/2 producers). Soultz is in the Alsace.

Induced seismicity in Basel led to the cancellation of the EGS project there.

The Portuguese government awarded, in December 2008, an exclusive license to Geovita Ltd to prospect and explore geothermal energy in one of the best areas in continental Portugal. Geovita is studying an area of about 500 square kilometers together with the Earth Sciences department of the University of Coimbra's Science and Technology faculty.

The Pohang EGS project started in December 2010, with the goal of producing 1 MW.[76]

The 2017 Pohang earthquake may have been linked to the activity of the Pohang EGS project. All research activities were stopped in 2018.

The first EGS effort — then termed Hot Dry Rock — took place at Fenton Hill, New Mexico with a project run by the federal Los Alamos Laboratory.[79] It was the first attempt to make a deep, full-scale EGS reservoir.

The EGS reservoir at Fenton Hill was completed in 1977 at a depth of about 2.6 km, exploiting rock temperatures of 185 °C. In 1979 the reservoir was enlarged with additional hydraulic stimulation and was operated for about 1 year. The results demonstrated that heat could be extracted at reasonable rates from a hydraulically stimulated region of low-permeability hot crystalline rock. In 1986, a second reservoir was prepared for initial hydraulic circulation and heat extraction testing. In a 30-day flow test with a constant reinjection temperature of 20 °C, the production temperature steadily increased to about 190 °C, corresponding to a thermal power level of about 10 MW. Budget cuts ended the study.

In 2009, The US Department of Energy (USDOE) issued two Funding Opportunity Announcements (FOAs) related to enhanced geothermal systems. Together, the two FOAs offered up to $84 million over six years. [80]

The DOE opened another FOA in 2009 using stimulus funding from the American Reinvestment and Recovery Act for $350 million, including $80 million aimed specifically at EGS projects,[81]

Developing EGS in conjunction with a district heating system is a part in Cornell University's Climate Action Plan for their Ithaca campus.[84] The project began in 2018 to determine feasibility, gain funding and monitor baseline seismicity.[85] The project received $7.2 million in USDOE funding.[86] A test well was to be drilled in spring of 2021, at a depth of 2.5 –5 km targeting rock with a temperature > 85 °C. The site is planned to supply 20% of the campus' annual heating load. Promising geological locations for reservoir were proposed in the Trenton-Black River formation (2.2 km) or in basement crystalline rock (3.5 km).[87] The 2 mile deep borehole was completed in 2022.[88]

In September 2022, the Geothermal Technologies Office within the Department of Energy's Office of Energy Efficiency and Renewable Energy announced an "Enhanced Geothermal Shot" as part of their Energy Earthshots campaign.[89] The goal of the Earthshot is to reduce the cost of EGS by 90%, to $45/megawatt hour by 2035.[90]

The Infrastructure Investment and Jobs Act authorized $84 million to support EGS development through four demonstration projects.[91] The Inflation Reduction Act extended the production tax credit (PTC) for renewable energy sources (including geothermal) until 2024 and included geothermal energy in the new Clean Electricity PTC to begin in 2024.[92]

Geothermal power technologies.

A 2006 report by MIT,[8] funded by the U.S. Department of Energy, conducted the most comprehensive analysis to date on EGS. The report offered several significant conclusions:

• Resource size: The report calculated United States total EGS resources at 3–10 km of depth to be over 13,000 zettajoules, of which over 200 ZJ were extractable, with the potential to increase this to over 2,000 ZJ with better technology.[8] It reported that geothermal resources, including hydrothermal and geo-pressured resources, to equal 14,000 ZJ — or roughly 140,000 times U.S. primary energy use in 2005.
• Development potential: With an R&D investment of $1 billion over 15 years, the report estimated that 100 GWe (gigawatts of electricity) or more could be available by 2050 in the United States. The report further found that "recoverable" resources (accessible with today's technology) were between 1.2 and 12.2 TW for the conservative and moderate scenarios respectively.
• Cost: The report claimed that EGS could produce electricity for as low as 3.9 cents/kWh. EGS costs were found to be sensitive to four main factors:
  1. Temperature of the resource
  2. Fluid flow through the system
  3. Drilling costs
  4. Power conversion efficiency

The idea of deep hot dry rocks heat mining was described by Konstantin Tsiolkovsky (1898), Charles Parsons (1904), and Vladimir Obruchev (1920).[103]

In 1963 in Paris, a geothermal heating system that used the heat of natural fractured rocks was built.[103]

The Fenton Hill project was the first system for extracting HDR geothermal energy from an artificial formed reservoir; it was created in 1977.[103]

As the reservoir is formed by the pressure-dilation of the joints, the elastic response of the surrounding rock mass results in a region of tightly compressed, sealed rock at the periphery—making the HDR reservoir totally confined and contained. Such a reservoir is therefore fully engineered, in that the physical characteristics (size, depth at which it is created) as well as the operating parameters (injection and production pressures, production temperature, etc.) can be pre-planned and closely controlled. On the other hand, the tight compression and confined nature of the reservoir severely limits that amount and the rate at which energy can be extracted.

As described by Brown,[104] an HDR geothermal energy system is developed, first, by using conventional drilling to access a region of deep, hot basement rock. Once it has been determined the selected region contains no open faults or joints (by far the most common situation), an isolated section of the first borehole is pressurized at a level high enough to open several sets of previously sealed joints in the rock mass. By continuous pumping (hydraulic stimulation), a very large region of stimulated rock is created (the HDR reservoir) which consists of an interconnected array of joint flow paths within the rock mass. The opening of these flow paths causes movement along the pressure-activated joints, generating seismic signals (microearthquakes). Analysis of these signals yields information about the location and dimensions of the reservoir being developed.

Typically, an HDR reservoir forms in the shape of an ellipsoid, with its longest axis orthogonal to the least principal Earth stress. This pressure-stimulated region is then accessed by two production wells, drilled to intersect the HDR reservoir near the elongated ends of the stimulated region. In most cases, the initial borehole becomes the injection well for the three-well, pressurized water-circulating system.

In operation, fluid is injected at pressures high enough to hold open the interconnected network of joints against the Earth stresses, and to effectively circulate fluid through the HDR reservoir at a high rate. During routine energy production, the injection pressure is maintained at just below the level that would cause further pressure-stimulation of the surrounding rock mass, in order to maximize energy production while limiting further reservoir growth. However, the limited reservoir size limits reservoir energy. Meanwhile, high pressure operation adds significant cost to piping and pumping systems.

The volume of the newly created array of opened joints within the HDR reservoir is much less than 1% of the volume of the pressure-stimulated rock mass. As these joints continue to pressure and cooling -dilate, the overall flow impedance across the reservoir is reduced, leading to a high thermal productivity. If the cooling leads to cooling fractures in a way that exposes more rock then it is possible that these reservoirs may improve over time. To date reservoir energy growth is only reported to come from new expensive high pressure well stimulation efforts.

The feasibility of mining heat from the deep Earth was proven in two separate HDR reservoir flow demonstrations—each involving about one year of circulation—conducted by the Los Alamos National Laboratory between 1978 and 1995. These groundbreaking tests took place at the Laboratory's Fenton Hill HDR test site in the Jemez Mountains of north-central New Mexico, at depths of over 8,000 ft (2,400 m) and rock temperatures in excess of 180 °C.[105] The results of these tests demonstrated conclusively the engineering viability of the revolutionary new HDR geothermal energy concept. The two separate reservoirs created at Fenton Hill are still the only truly confined HDR geothermal energy reservoirs flow-tested anywhere in the world.

The first HDR reservoir tested at Fenton Hill, the Phase I reservoir, was created in June 1977 and then flow-tested for 75 days, from January to April 1978, at a thermal power level of 4 MW.[106] The final water loss rate, at a surface injection pressure of 900 psi (6.2 MPa), was 2 US gallons per minute (7.6 L/min) (2% of the injection rate). This initial reservoir was shown to essentially consist of a single pressure-dilated, near-vertical joint, with a vanishingly small flow impedance of 0.5 psi/US gal/min (0.91 kPa/L/min).

The initial Phase I reservoir was enlarged in 1979 and further flow-tested for almost a year in 1980.[107] Of greatest importance, this flow test confirmed that the enlarged reservoir was also confined, and exhibited a low water loss rate of 6 gpm. This reservoir consisted of the single near-vertical joint of the initial reservoir (which, as noted above, had been flow-tested for 75 days in early 1978) augmented by a set of newly pressure-stimulated near-vertical joints that were somewhat oblique to the strike of the original joint.

A deeper and hotter HDR reservoir (Phase II) was created during a massive hydraulic fracturing (MHF) operation in late 1983.[107] It was first flow-tested in the spring of 1985, by an initial closed-loop flow test (ICFT) that lasted a little over a month.[108] Information garnered from the ICFT provided the basis for a subsequent long-term flow test (LTFT), carried out from 1992 to 1995.

The LTFT comprised several individual steady-state flow runs, interspersed with numerous additional experiments.[109] In 1992–1993, two steady-state circulation periods were implemented, the first for 112 days and the second for 55 days. During both tests, water was routinely produced at a temperature of over 180 °C and a rate of 90–100 US gal/min (20–23 m³/h), resulting in continuous thermal energy production of approximately 4 MW. Over this time span, the reservoir pressure was maintained (even during shut-in periods) at a level of about 15 MPa.

Beginning in mid-1993, the reservoir was shut in for a period of nearly two years and the applied pressure was allowed to drop to essentially zero. In the spring of 1995, the system was re-pressurized and a third continuous circulation run of 66 days was conducted.[110] Remarkably, the production parameters observed in the two earlier tests were rapidly re-established, and steady-state energy production resumed at the same level as before. Observations during both the shut-in and operational phases of all these flow-testing periods provided clear evidence that the rock at the boundary of this man-made reservoir had been compressed by the pressurization and resultant expansion of the reservoir region.

As a result of the LTFT, water loss was eliminated as a major concern in HDR operations.[111] Over the period of the LTFT, water consumption fell to just 7% of the quantity of water injected; and data indicated it would have continued to decline under steady-state circulation conditions. Dissolved solids and gases in the produced fluid rapidly reached equilibrium values at low concentrations (about one-tenth the salinity of sea water), and the fluid remained geochemically benign throughout the test period.[112] Routine operation of the automated surface plant showed that HDR energy systems could be run using the same economical staffing schedules that a number of unmanned commercial hydrothermal plants already employ.

An advantage of an HDR reservoir is that its confined nature makes it highly suitable for load-following operations, whereby the rate of energy production is varied to meet the varying demand for electric power—a process that can greatly increase the economic competitiveness of the technology.[113]

In 1986 the HDR system project of France and Germany in Soultz-sous-Forêts was started. In 1991 wells were drilled to 2.2 km depth and were stimulated. However, the attempt to create a reservoir was unsuccessful as high water losses was observed.[114][8]

In 1995 wells were deepened to 3.9 km and stimulated. A reservoir was created successfully in 1997 and a four-month circulation test with 25 L/s (6.6 USgal/s) flow rate without water loss was attained.[8]

In 2003 wells were deepened to 5.1 km. Stimulations were done to create a third reservoir, during circulation tests in 2005-2008 water was produced at a temperature of about 160 °C with low water loss. Construction of a power plant was begun.[115] The power plant started to produce electricity in 2016, it was installed with a gross capacity of 1.7 MW_e.[116]

There have been numerous reports of the testing of unconfined geothermal systems pressure-stimulated in crystalline basement rock: for instance at the Rosemanowes quarry in Cornwall, England;[117] at the Hijiori[118] and Ogachi[119] calderas in Japan; and in the Cooper Basin, Australia.[120] However, all these “engineered” geothermal systems, while developed under programs directed toward the investigation of HDR technologies, have proven to be open—as evidenced by the high water losses observed during pressurized circulation.[121] In essence, they are all EGS or hydrothermal systems, not true HDR reservoirs.

The EGS concept was first described by Los Alamos researchers in 1990, at a geothermal symposium sponsored by the United States Department of Energy (DOE)[122]—many years before the DOE coined the term EGS in an attempt to emphasize the geothermal aspect of heat mining rather than the unique characteristics of HDR.

Hot Wet Rock (HWR) hydrothermal technology makes use of hot fluids found naturally in basement rock; but such HWR conditions are rare.[123] By far the bulk of the world's geothermal resource base (over 98%) is in the form of basement rock that is hot but dry—with no naturally available water. This means that HDR technology is applicable almost everywhere on Earth (hence the claim that HDR geothermal energy is ubiquitous). On the other hand, an uneconomic resource is actually just energy storage and not useful.

Typically, the temperature in those vast regions of the accessible crystalline basement rock increases with depth. This geothermal gradient, which is the principal HDR resource variable, ranges from less than 20 °C/km to over 60 °C/km, depending upon location. The concomitant HDR economic variable is the cost of drilling to depths at which rock temperatures are sufficiently high to permit the development of a suitable reservoir.[124] The advent of new technologies for drilling hard crystalline basement rocks, such as new PDC (polycrystalline diamond compact) drill bits, drilling turbines or fluid-driven percussive technologies (such as Mudhammer [125]) may significantly improve HDR economics in the near future.

A definitive book on HDR development, including a full account of the experiments at Fenton Hill, was published by Springer-Verlag in April 2012.[105]

• DOE, Department of Energy (United States)
• EGS, Enhanced geothermal system
• HDR, Hot dry rock
• HWR, Hot wet rock
• ICFT, Initial closed-loop flow test
• LTFT, Long-term flow test
• MHF, Massive hydraulic fracturing
• PDC, Polycrystalline diamond compact (drill bit)